System and method for control of side layer formation in an aluminium electrolysis cell

ABSTRACT

A system and method is provided for control of layer formation by use of sidelining provided with heat tube.

BACKGROUND OF THE INVENTION

1. Field of the invention

The invention regards heat regulation in general and particularly method and system for use for control of layer formation in an aluminium electrolysis cell and exploitation of heat.

1. Background information

During production of aluminium with electrolysis technology of today based on so called Hall-Heroult cells, the operations of the cells depend on the formation and maintenance of a protective layer of frozen electrolyte in the side walls of the cell. This frozen bath is called side layer and protects the side lining of the cells against chemical and mechanical wear, and is an essential condition for achieving long lifetime of the cells. The crystallized bath operates simultaneously as a buffer for the cell with regards of changes in the heat balance. During operations the heat generation and the heat balance of the cell will vary due to unwanted disturbances of the operation (changes in bath acidity, changes in alumina concentration, changes in interpolar distances, etc.) and desired activities on the cells (metal tapping, change of anode, fire, etc.). This causes the thickness of the layer of the periphery of the cell to change and in some cases the layer will disappear entirely in parts of the periphery. Then the side lining will be exposed against the electrolyte and metal, which in combination with oxidizing gasses will lead to corrosion of the side lining materials causing these to erode. During operations over time run outs in the side can result from such repeated occurrences. It is therefore of importance to control formation of layer and layer stability in Hall-Heroult cells. For Hall-Heroult cells with high current densities model calculations show that it will be difficult to maintain the side layer of the cell due to large heat generation. For such cells and for traditional cells with heat balance problems it will therefore be a condition for a long life cell that one is able to maintain the layer protecting the side lining.

During production of aluminium in accordance with Hall-Heroult principle, this takes place at present with relatively high use of energy as measured in kilo watt hours per kilo aluminium. The heat generation of the electrolysis cells takes place as a result of ohmic voltage drops in the cell, for instance in current feeds, produced metal and particularly in the electrolyte. Approximately 55% of input energy to the electrolysis cell is used for heat generation in the cell. Data from literature indicates that approximately 40% of the total heat loss from the cells is lost through the side lining. Due to the high heat loss and the protecting frozen layer in the side lining it is a preferable place to place elements for heat regeneration in this area of the cell.

There is a desire for optimizing control of layer formation and heat regeneration. In order to optimize both of this purpose at the same time it is important that heat regeneration takes place as close to the formed side layer as possible. This will lead to the control of and speed on layer formation is as fast as possible, and that temperature difference between input and output cooling medium is as large as possible. The latter is preferable for exploitation/regeneration of energy.

From the known art one should refer to granted patent NO 318012, brought into the PCT-phase as PCT/NO2004/000070. This describes a sidelining formed with hollows for flow-through of a cooling medium. The manufacturing process of this, however, is complex and requires the side linings to be moulded with hollows formed preferably before the material is sintered.

In general it is a problem that efficient heat transfer requires small and thin canals, however these are difficult to manufacture in a reliable fashion and can be blocked during sintering.

Furthermore one should refer to GB 2076428A describing an aluminium cell with isolating layers in bottom and walls. In the walls there are carbon blocks with heat tubes moulded in for the removal of heat from the cell. The heat tubes can be adjusted with different distances in order to vary the degree of heat removal.

The problem with heat tubes brought into holes drilled into a sidelining material is that the difference between thermal coefficient of expansion for heat tube and sidelining material can lead to formation of cracks in the sidelining or that the heat tube loses contact with the sidelining. There will also be a significant temperature difference along the hole in the side lining.

There is therefore a need for a method and a system overcoming the above mentioned problems.

OBJECTS OF THE INVENTION

The object of the invention is to provide a method and system for use for control of layer formation in an aluminium electrolysis cell and exploitation of the heat.

SUMMARY OF THE INVENTION

The invention provides thus a system for use for control of layer formation in an aluminium electrolysis cell and exploitation of heat comprising sidelining provided with at least one hollow for heat transfer and at least one heat tube, characterized in that the heat tube is provided by the hollow and that the hollow is at least one canal provided along the surface of the sidelining.

There is also provided a method for control of layer formation in an aluminium electrolysis cell and exploitation of heat comprising sidelining provided with at least one hollow for heat transfer and with at least one heat tube, wherein the heat tube is provided by the hollow and that the hollow is at least one canal provided along the surface of the sidelining, characterized in conducting the heat away using said at least one heat tube.

Beneficial and preferable embodiments of the invention are stated in the dependent claims.

In accordance with the present invention a sidelining provided with heat tube for transport of heat is provided.

The present invention relates to structural elements for forming a sidelining material for cooling of sidelining in aluminium electrolysis cells for the purpose of controlling and adjusting sidelining thickness in the cell. By the chosen form of the sidelining materials it is also possible to provide heat exchange of such cells with possibility of regeneration of heat as electrical energy and/or heat. By the forming of sidelining materials in the present invention one should understand design, formation and production of hollows in the material for the purpose of mounting of heat tubes such as heat pipes to lead heat to the outside of the aluminium electrolysis bath. The evaporation end of the heat tube is mounted on the inside of the electrolysis bath and is in contact with the above mentioned sidelining material while on the heat tube condensation end a cooling element is mounted transferring the transported heat to a suited cooling medium such as for instance oil.

MEANS FOR SOLVING THE PROBLEMS The present invention achieves the objectives outlined above by a sidelining as described in claim 1 and a method as described in claim 4.

The sidelining material is manufactured without requirements for inserting thin canals. Instead a plurality of heat tubes are utilized, attached to the sidelining.

The technical effect of the difference is that one achieves efficient transfer of heat without having to sinter sidelinings with thin canals along with the problems that this incurs. At the same time heat tubes can be more efficient in transferring heat than use of cooling medium in the canals. A further technical effect is that the heat tubes will be more stably fastened to the longitudinal canals in the surface of the sidelining and that there will be a low thermal gradient along the canal compared to canals known from the state of the art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a typical embodiment of the invention in the form of a sidelining block with hollows and heat tube for an electrolysis cell,

FIG. 2 shows a detail section of the embodiment of FIG. 1 together with section as seen from the side,

FIG. 3 shows an example of providing heat tubes in the sidelining block,

FIG. 4 shows another example of providing heat tubes in the sidelining block.

The following reference numerals and signs refer to the drawings:

 1 Anode hanger  2 Anode carbon block  3 Liquid electrolyte  4 Liquid aluminium  5 Cathode carbon  6 Frozen electrolyte  7 Isolating brickwork  8 Steel shell  9 Ramming paste 10 Heat isolation 11 Sidelining block 12 Heat tube 13 Condensation unit for heat tube 14 Condensation fins 11a SiC-block - example a 12a Heat tube - example a 13a Condensation unit for heat tube - example a 11b SiC-block - example b 12b Heat tube - example b 13b Condensation unit for heat tube - example b

DETAIL DESCRIPTION OF THE INVENTION

With sidelining one should here understand this to mean sidelining block 11 together with the heat isolation 10, wherein the sidelining block is provided with heat tube 12. The sidelining block 11 is typically made from SiC 11 a, 11 b as in example a and b.

The invention will in the following be described in more details with is references to the drawings showing embodiments and where FIG. 1 schematically shows a typical embodiment of the invention in the form of a sidelining block with hollows and heat tubes for an electrolysis cell.

One of the main components of the invention is the sidelining block is manufactured from a ceramic material. It is manufactured in a particular way to achieve the intended hollows. One example of possible positioning of hollows are shown in FIG. 1 with a detail section in FIG. 2, but also other forms can be used for the hollows. FIG. 2 shows the detail section also from the side where the heat tubes are positioned standing along the canals in the surface of the sidelining block.

Examples of other embodiments are shown in FIG. 3 and in FIG. 4 with inclined heat tubes.

Heat tubes are positioned against these hollows and an example of such a heat tube in the form of a heat pipe.

On the cold end of the heat tube, also known as the condensation side, a cooling element is mounted comprising a condensation unit for heat tube 13 and condensation fins 14.

In the figures one can find two small tube ends that are mounted on the is condensation unit 13. One end is meant as an input while the other is an output for the cooling medium that is to remove heat from the heat tube. Such cooling elements can be connected together.

PRINCIPLES FORMING THE BASIS OF THE INVENTION

The invention achieves its solution by the assembly of plurality of principles.

One absolute requirement is that the heat loss through the sidelining assures building up of a sufficient layer of side layer. According to the invention this is assured using heat tube that efficiently transports large amounts of heat energy out of the side lining.

Another principle is the use of heat tube where the phase transition liquid to vapour in the hot end transports large amounts of heat to the cold end where the vapour condenses to liquid which then is returned to the hot end. By heat tube there are two embodiments intended: “heat pipe” where a wick or other capillary effect pulls the liquid back to the hot end, and “thermosyphon” where the gravity pulls the liquid back to the hot end.

PREFERRED EMBODIMENT

An aluminium electrolysis cell will comprise several tens of such “heat pipe” heat exchangers in the sidewalls. The heat removed from the cells will be transported in a cooling medium. This heat can for instance be exploited to produce electrical energy. A plurality of electrolysis cells can also be connected together in order to regenerate the cooled effect in an efficient manner.

INDUSTRIAL APPLICABILITY

The invention is applicable for control of layer formation in a aluminium electrolysis cell and exploitation of the heat. 

1. System for use for control of layer formation in an aluminium electrolysis cell and exploitation of heat comprising sidelining (10, 11) provided with at least one hollow for heat transfer and at least one heat tube (12), characterized in that the heat tube (12) is provided by the hollow and that the hollow is at least one canal provided along the surface of the sidelining (10, 11).
 2. System according to claim 1, characterized in that the heat tube (12) is a heat pipe.
 3. System according to claim 1, characterized in that the heat tube (12) is a thermosyphon.
 4. Method for control of layer formation in an aluminium electrolysis cell and exploitation of heat comprising sidelining (10, 11) provided with at least one hollow for heat transfer and with at least one heat tube (12) wherein the heat tube (12) is provided by the hollow and that the hollow is at least one canal provided along the surface of the sidelining (10, 11), characterized in conducting the heat away using said at least one heat tube. 